Pressure transducers have been employed in a myriad of applications. One such transducer is the capacitive manometer which provides very precise and accurate measurements of pressure of a gas, vapor or other fluid. Applications include high-precision gas and vapor delivery systems, which have become very important in many industrial applications, for example in the semiconductor industry for wafer and chip fabrication, although other applications are known. Such fluid delivery systems typically include, but are not limited to, devices such as mass flow controllers (MFCs) and mass flow verifiers (MFVs) for regulating and/or monitoring the flow of gases and vapors.
Capacitive manometers typically use (a) a flexible diaphragm forming or including an electrode structure and (b) a fixed electrode structure spaced from the diaphragm so as to establish capacitance there between. Variations in pressure on one side of the diaphragm relative to the pressure on the opposite side of the diaphragm causes the diaphragm to flex so that the capacitance between the electrode structure of the diaphragm and the fixed electrode structure varies as a function of this differential pressure. Usually, the gas or vapor on one side of the diaphragm is at the pressure being measured, while the gas or vapor on the opposite side of the diaphragm is at a known reference pressure, whether at atmosphere or some fixed high or low (vacuum) pressure, so that the pressure on the measuring side of the diaphragm can be determined as a function of the capacitance measurement.
Many applications requiring extremely low vacuum pressures have been and continue to be developed resulting in the need for capacitive manometers capable of measuring such low vacuum pressures. However, increasing the sensitivity of capacitive manometers to provide very accurate pressure measurements at low vacuum pressures poses several design challenges. In order to measure extremely low vacuum pressures, capacitive manometers require very narrow gaps between the flexible diaphragm and the fixed electrode structure so that they can detect small changes in pressure. Such narrow gaps establish a relatively small predefined base capacitance when the diaphragm is in the zero position. Consequently, a relatively small flexure of the diaphragm due to a small change in differential pressure will provide a detectable change in capacitance, known as stroke capacitance. However, making the gap smaller to make the manometer more sensitive increases the chances that over pressurization can occur causing the diaphragm electrode to directly contact the opposing electrode structure. The contact results in an electrical short between the opposing electrode components, with an increase in power usage causing heat to be generated by both the contacting electrode components and the elements used to deliver and regulate the power to the electrode components. The heat can cause the diaphragm to thermally expand, distorting the diaphragm so that upon relaxation of the diaphragm (under conditions where the pressure is the same on both sides of the diaphragm), the diaphragm returns to an incorrect non-zero position, and only slowly creeps back to the zero position as the diaphragm returns to its proper temperature.